For many years inorganic fibers, generically referred to in the industry as "mineral wool fibers", made from slag, rock, fly ash, and other by-product raw materials have been manufactured. These fibers have been typically manufactured by melting the slag, rock, etc., containing such oxides as silica, alumina, iron oxide (ferrous and ferric), calcium oxide, and magnesia; allowing the molten material to be blown by gas or steam or to impinge on rotors at high speeds; and causing the resulting blown or spun fibers to be accumulated on a collecting surface. These fibers are then used in bulk or in the form of mats, blankets, and the like as both low and high temperature insulation. U.S. Pat. No. 2,576,312 discloses a conventional mineral wool composition and method for making the same.
In the past, the industry has well recognized the standard drawbacks associated with conventional mineral wool fibers. Conventional mineral wool fibers may have high contents of undesired oxides which often detract from their refractory properties. The conventional mineral wools are coarse, i.e. they have average fiber diameters of 4 to 5 microns (measured microscopically) and have high shot contents in the range of 30 to 50 weight percent. The coarseness of the fiber reduces the insulating value of the fiber and makes conventional mineral wool unpleasant to handle and unfriendly to the touch. For example, because of their coarse fiber diameters, conventional mineral wool blankets must have bulk densities of from 4 to 8 pcr and even higher in order to pass the ASTM E-119 two hour fire test. On the other hand, fiber glass blankets are often made with bulk densities of 2 pcf or lower. While the fiber glass blankets are friendly because of their low bulk densities and relatively fine fiber diameter, they do not have sufficient fire resistance so as to pass even the one hour ASTM E-119 fire test.
Recently, another potential problem with traditional mineral wool and other types of fiber has been recognized. It is well known that inhalation of certain types of fiber can lead to elevated incidence of respiratory disease, including cancers of the lung and surrounding body tissue. Several occurrences are well-documented in humans for several types of asbestos fiber. Although for other varieties of natural and manmade mineral fiber direct and unequivocal evidence for respiratory disease is lacking, the potential for such occurrence has been inferred from results of tests on laboratory animals. In the absence or insufficiency of direct human epidemiclogical data, results from fiber inhalation or implantation studies on animals provides the best "baseline information" from which to extrapolate disease potential.
Chronic toxicological studies on animals have, however, been able to statistically demonstrate the importance of three key factors that relate directly to the potential for respiratory disease and especially carcinoma: (a) dose of fiber received (including time of exposure); (b) dimension of the inhaled fiber; and (c) persistence of the fiber within the lung. The effects of dose and dimension have been well-characterized from such studies and as a result are fairly well known in regard to human disease potential. The dose is obviously a product of the environment in which the fiber is used and the manner in which it is used. The dimension and persistence of the fiber within the lung, on the other hand, are functions of the manner in which the fiber is formed and of its chemical composition. In general, the smaller the fiber the more likely that it will become embedded in lung tissue when inhaled, thus increasing the danger of respiratory disease.
Although less is known about the link between persistence of the fiber within the lung and respiratory disease, increasing attention is being focused on this aspect of the health issue. Biological persistence refers to the length of time a fiber endures as an entity within the body. The physiochemical concept that most closely relates to persistence and is perhaps more easily quantified is that of "durability"--specifically, the chemical solubility (or resistance to solubility) of fibers in body fluids and the tendency of such fibers to maintain physical integrity within such an environment. In general, the less durable a fiber is, the less will be the potential health risk associated with the inhalation of that fiber. One method of measuring the chemical durability of a fiber in body fluids is to measure its durability in physiological saline solutions. This can be done by quantifying the rate of extraction of a chemical component of the fiber such as silicon into the physiological saline solution over a certain period of time.
Thus, as can be easily concluded from the foregoing discussion, conventional mineral wool fibers have several serious drawbacks. However, even the alternatives to mineral wools have problems. For example, as mentioned earlier glass fibers have a fire resistance problem and whereas the refractory ceramic fibers have been gaining increasing use in recent years as an alternative to mineral wool fibers because of their ultra-high temperature resistance and superior ability to pass all fire rating tests, their use is limited by the fact that they are relatively expensive and have a relatively high chemical durability in physiological saline solutions as well.
In conclusion, there is a great need in the industry for low cost, friendly feeling low bulk density inorganic fibers which have good fire resistance properties as measured by their ability to pass the ASTM E-119 two hour fire test. Additionally, there is a tremendous demand for fibers which have especially low durabilities in physiological saline solutions. What would be particularly advantageous to the industry would be fibers with combinations of the above mentioned sought after properties. Also advantageous would be fibers which also have excellent refractory properties as well, e.g. high continuous service temperatures.